When mobile telephone systems were first developed, circuit switched (CS) techniques were used. An example of a CS mobile telephone standard is GSM. In parallel, computer communications developed on the basis of a variety of protocols and most notably the Internet Protocol (IP), which is a packet based form of communication. A convergence of computer communication and mobile telephony has led to the introduction of the so-called IP Multimedia Subsystem (IMS) in mobile communication systems, which is an IP based network for providing certain IP based services to mobile subscribers.
More recently, the concept of IMS Centralized Services (ICS) has been proposed, see e.g. 3GPP TS 23.292 V.9.4.0 of December 2009. With ICS, user services are provided by the IMS. ICS subscribers may access the IMS in a number of ways, i.e. by means of different access paths, e.g. via a packet switched (PS) access (such as GPRS or UMTS), but also may connect via a CS connection. Thus, a user equipment (UE) may register or attach to IMS in a CS domain. The CS domain will comprise a Mobile Switching Center (MSC) or an MSC server. The MSC or MSC server receives subscriber data from a Home Subscriber Server (HSS) or Home Location Register (HLR) after a UE and MSC server perform a location update procedure. The subscriber data may include an optional flag. If the MSC server is enhanced for ICS it analyzes the flag value and attempts IMS registration if the flag indicates an ICS subscription, and otherwise does not attempt registration. If the MSC server is not enhanced for ICS it ignores the flag and performs regular or normal CS operation, i.e. it provides CS services as defined or standardized for CS (e.g. GSM) subscribers.
An MSC server enhanced for ICS needs to be equipped to act as a gateway to the IMS, e.g. has to provide conversion between the signalling protocols for call control on the CS side and the IMS side.
Migration of the CS subscriber base to IMS is an important issue for mobile operators. How to migrate millions of CS users to IMS in a cost efficient, practical and secure way is a concern in this connection. Some envisionable concepts are the use of Single Radio Voice Call Continuity (SR-VCC) and CS Fallback (CSFB), but application in multi-vendor networks (i.e. networks in which different network components are provided by different vendors) can be difficult. Multi-vendor networks are, however, the rule real mobile networks. A further difficulty is that there are cases where a CS vendor may not be willing to support the above mentioned migration features, or is not able to implement them in a timeframe as required by the migration plans of the operator.
Currently there is no practical solution of how to support IMS migration in a mixed vendor network where the supported services do not match. Specifically, there are cases where one vendor supports a proprietary set of legacy services, which cannot be migrated to IMS service domain, and at the same time this vendor does not support the above mentioned IMS migration features.
In a completely different context than IP based systems and subscription migration to such networks, it is known to provide pooling of MSCs, i.e. a concept where one access network may be shared by a plurality of core networks (CN), such that an access network node is not fixedly associated with a CN node, and thus a mechanism is provided for letting an access network select among CN nodes. EP 1 280 365 B1 describes such a system, including a mechanism for redirecting subscribers from one CN node to another.